The grain oriented silicon steel sheets are required to have high magnetic flux density and a low iron loss as magnetic properties. With recent progress in production techniques, for example, 0.23 mm thick steel sheets having a magnetic flux density B.sub.8 (value at 800 A/m of magnetizing forces) being 1.92 T are obtained, and products having excellent iron loss property W.sub.17/50 (value at the maximum magnetization of 1.7 T under 50 Hz) being 0.90 W/kg can be produced in an industrial scale.
The materials having such excellent magnetic properties comprise crystalline structure in which &lt;001&gt; orientation as an axis of easy magnetization of iron is highly arrayed in a rolling direction of the steel sheet. A texture of such a crystalline structure is formed by a phenomenon called secondary recrystallization in which crystalline grains having (110)[001]called Goss orientation preferentially vigorously grow during final finish annealing in the production of the grain oriented silicon steel sheets. As a fundamental factor required to sufficiently grow these secondary recrystallized grains having the (110)[001]orientation, it is a well known fact that an inhibitor must be present to control growth of crystalline grains having unfavorable orientations other than the (110)[001]orientation during the secondary recrystallization step, and that the primary recrystallized structure is present for favorably sufficiently growing the secondary recrystallized grains having the (110)[001]orientation.
In general, a finely precipitatable material of MnS, MnSe, AlN or the like is used as an inhibitor. Further, it is common practice that effects of the 0 inhibitor are strengthened by adding a grain boundary segregatable type element such as Sb, Sn or the like together in combination as disclosed in Japanese patent publications Nos. 51-13,469 or 54-32,412.
On the other hand, in order to form an appropriate primary recrystallized structure, various countermeasures have heretofore been taken in each of hot rolling and cold rolling. For example, as to the strongly cold rolling using AlN as inhibitor, it is considered particularly effective to impart thermal effects at the time of warm rolling or cold rolling such as interpass aging as disclosed in Japanese patent publication Nos. 50-26,493, 54-13,846 and 54-29,182. This technique is to form a favorable texture by changing the deforming mechanism of the materials on rolling through utilization of interaction among dislocation and N and C as elements solid-solved in steel.
However, it is hard to say that the above prior art techniques are advantageous processes in view of the productivity. Moreover, good magnetic properties cannot always be stably obtained by these techniques. For example, the processes are technically difficult to carry out on an industrial scale in the case of warm rolling. On the other hand, interpass aging is ordinarily effected by thermally treating the coiled steel sheet at the number of plural times with using a reversing mill having one stand. The reason is that the steel sheet cannot uniformly thermally be treated over the entire coil length in the coiled state.
Incidentally, in order to enhance productivity, techniques using tandem mills comprising a plurality of stands have recently become the main trend. Different from the reversing mill, proportions of the draft among the passes must match with the rolling speed in the case of the rolling by using the tandem mill. Consequently, the deformation is naturally mainly compression deformation rather than tension deformation. Therefore, since the deformation mechanism in the rolling greatly differs from that in the prior art techniques, no satisfactory effect can be obtained by conventional aging treatment. This is a barrier in the case of tandem-rolling silicon steel sheets having high magnetic flux density and containing Al. In addition, repeated aging treatment conspicuously deteriorates productivity in view of the character of the tandem rolling. Therefore, there remains a problem in that aging cannot be effected at the number of plural times unlike the prior art techniques to enhance the aging effects.